Use As.EXISTING_PROPERTY not to generate JSON with duplicate keys

[Yang-33]

Hi,

We found com.linecorp.bot.model.narrowcast.recipient generated JSON with duplicate keys.
For example, audience class is serialized like below.

{
    "type":"audience",
    "audienceGroupId": 123,
    "type":"audience"
}

FasterXML/jackson-databind#528 reported this. The audience class has type filed and generates duplicate "type": "audience" if this class doesn't have @JsonTypeInfo(include = JsonTypeInfo.As.EXISTING_PROPERTY).
Other models don't have type filed, so it's enough to add this config to only this class.

About JSON format, https://www.rfc-editor.org/rfc/rfc7159#section-4 says

... The names within an object SHOULD be unique. ...

So this change follows JSON format the RFC defines.

Thanks!

[tokuhirom]

Good catch, thanks!